The present invention relates to meters, and more particularly relates to meters having load control units configured to interrupt or shed loads at a customer location.
Loads at a customer location may be curtailed or interrupted during power system events for several reasons. Depending on the driver, the load reduction may be initiated by the utility or by the customer. For the customer, load reduction is generally an attempt to decrease energy consumption during certain periods of time to reduce costs. Predetermined loads such as HVAC, hot water heaters, pool pumps, or other high consumption devices can be selected for energy interruption to reduce the overall consumption when higher energy prices are in effect.
For the utility, load shedding at a customer site may be accomplished because of an abnormal condition on the power network. Abnormal conditions include events such as loss of transmission capability due to a line outage, loss of generation, loss of inter-tie to adjacent power networks, unusually high peak demand or similar type events. In this case the utility may take different steps to decrease consumption via load shedding. Load shedding can be initiated at the substation level using frequency-based relays. Many customers may contractually elect to have non-critical loads interrupted at customer premises for a reduction in overall energy costs. In order for this to be effective, the utility traditionally interrupts certain loads for a few minutes to maybe hours in order to reduce the overall load on a transmission network.
Conventional switch systems of the type illustrated in FIG. 1 provide an electronic circuit 20 that includes a dual-coil solenoid switch 22 fed from a DC supply voltage 24 on a positive supply line 26. The solenoid coils 22a, 22b can be energized with a current that causes an associated movable member (not shown) to move between a “connect” and “disconnect” position with respect to the load. The circuit 20 includes MOSFET devices 28 and 30 that are operable to ground either one of the solenoid coils independently, thereby allowing current to flow through the associated solenoid coil to a ground 31. Specifically, turning one of the MOSFET devices “on” causes current to flow through its respective solenoid coil to cause the movable member of the switch to move to one of the positions (i.e., either “connect” or “disconnect”). Turning that MOSFET device “off” and then turning the other MOSFET device “on” causes current to flow through the other respective solenoid coil to cause the movable member of the switch to move to the other position. Thus, the connect/disconnect switch can be selectively activated by turning one of the MOSFET devices “on” while the other remains “off.”
Current flowing in an inductance coil (e.g., coil 22a or 22b) does not immediately dissipate to zero when the switch (e.g., MOSFET 28 or 30) in series with it begins to open. Depending on the inductor characteristics, the induction device may generate a high voltage associated with a quick rate of decrease in coil current (e=L di/dt), where “e” is the inductive voltage drop, “L” is the inductance of the coil, and “di/dt” is the rate of current change over time.
The inductive voltage can be sufficient to cause harm to the associated MOSFET device if an alternative path for current flow that bypasses the MOSFET device is not provided. Even if harm does not come to the associated MOSFET device, undesirable EMF noise can be developed due to the rapid rate of change of current.
Accordingly, in conventional connect/disconnect switch mechanisms such as that shown in FIG. 1, a pair of commutation diodes 32 and 34 is provided from the drain of each MOSFET to the positive supply line 26. The commutation diodes 32 and 34 are operable to provide a path for coil energy to be dissipated when a MOSFET device is switched off with current still flowing in the associated solenoid coil. When one of the MOSFET devices 28 or 30 stops conducting current, the inductance develops a reverse voltage equivalent to the diode voltage and the diode provides a path for a slow rate of decrease of current while preventing the development of potentially harmful high voltage conditions.
While electronic circuits of the type described above have proven useful for their intended purpose, electronic circuits providing for enhancements in load control are desirable.